The parent patent and the parent application both describe a plasma gun which may, among other things, be utilized to generate radiation in the EUV and soft x-ray bands with high reliability and at a PRF in excess of approximately 100 Hz, preferably in excess of 500 Hz, and preferably 1000 Hz or more for lithography and other applications requiring generation of such radiation. More specifically, the plasma gun of the parent application/patent involves a center electrode and an outer electrode substantially coaxial with the center electrode, a coaxial column being formed between the electrodes. A selected gas is introduced into the column through an inlet mechanism and a plasma initiator was provided at the base end of the column. A solid state high repetition rate pulse driver is provided which is operable on pulse initiation at the base of the column to deliver a high voltage pulse across the electrodes, the plasma expanding from the base of the column and off the end thereof. The pulse voltage and electrode lengths were selected such that the current for each voltage pulse is substantially at its maximum as the plasma exits the column. The outer electrode for this plasma gun embodiment is preferably the cathode electrode and may be solid or may be in the form of a plurality of substantially evenly spaced rods arranged in a circle. The inlet mechanism provides a substantially uniform gas fill in the column, resulting in the plasma being initially driven off the center electrode, the plasma being magnetically pinched as it exits the column to provide a very high temperature at the end of the center electrode. A selected gas/element fed to the pinch as part of the ionized gas, through the center electrode or otherwise, is ionized by the high temperature at the pinch to provide radiation at a desired wavelength. The wavelength is achieved by careful selection of various plasma gun parameters, including the selected gas/element fed to the pinch, current from the pulse driver, plasma temperature in the area of the pinch, and gas pressure at the column.
While radiation sources of the type indicated above, as described in far greater detail in the parent application and patent, can provide useful radiation at a desired wavelength, the high velocity of the plasma being driven down the column and off the center electrode can cause a problem which significantly limits the usefulness of such sources. In particular, temperatures at the pinch in the range of 100 eV (i.e., about 11,000.degree. C.) to 1000 eV, depending on the desired frequency of radiation, require magnetic compression fields which are sufficient to drive the plasma to velocities of several centimeters per microsecond. Plasmas moving at these velocities down the center conductor and off the end forming the pinch tend to continue moving out into space away from the end of the center conductor, the plasma sheath eventually losing electrical connection to the pinch. This prematurely ends the pinch after as little as 100 nanoseconds and also results in a large voltage transient in the thousands of volts range, resulting in a restrike which can severely damage the electrodes.
Since a discharge can last for several microseconds, if premature loss of electrical connection between the plasma sheath and the electrode could be eliminated, the pinch lifetime could be extended dramatically and the potentially damaging restrike eliminated. This could result in significantly increased output efficiency for the plasma source and a greatly expanded electrode lifetime for the source, thus reducing source down time and maintenance, both of which can be expensive in for example a lithographic application. Significantly better performance at lower costs can thus be obtained.
Further, while materials to be fed to the pinch to achieve certain wavelengths of output were suggested in the parent application, a specific material for providing radiation at the desirable one nanometer wavelength was not specifically indicated.